The loudspeaker amplifier in a line-powered loudspeaking telephone is easily excited to maximum level due to only limited power being available from the telephone network. This causes distortion, which is often already noticeable at normal speech levels and which can be heavy for high speech levels.
Simple and cheap loudspeaker elements usually have small diaphragm masses and small magnet systems, which give low mechanical attenuation and thereby a high Q-value. The efficiency of such loudspeaker elements is considerably higher for frequencies in the vicinity of the resonance frequency than for higher and lower frequencies. In the utilisation of such loudspeaker elements, a large part of the acoustic effect must therefore be attenuated with the aid of suitable absorbents in a sealed loudspeaker box so that the resulting efficiency will be relatively uniform. It will also be rather low, however. It is therefore generally the case in loudspeaking telephones to use expensive loudspeaker elements with relatively uniform efficiency. However, here also the elements must be mounted conventionally in a closed box with absorbents for attenuating resonances.